# HttpClient出現System.NullReferenceException

## 狀況
今天遇到一個問題，當我在使用這段程式碼時

private static HttpClient LoginWithJwtToken(string token)
{
    Client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", token);
    return Client;
}

var client = LoginWithJwtToken(token);
client.GetFromJsonAsync<List<FileInfoViewModel>>(string.Format(GetResourceFileListUrlFormatting, ApiUrlBase, resourceType.GetName()));

出現System.NullReferenceException，但是遇到這個問題的人零零散散
詳細錯誤如下：

System.NullReferenceException: Object reference not set to an instance of an object. at System.Net.Http.HttpConnection.WriteAsciiString(String s) at System.Net.Http.HttpConnection.WriteHeaderCollection(HttpHeaders headers, String cookiesFromContainer) at System.Net.Http.HttpConnection.WriteHeaders(HttpRequestMessage request, HttpMethod normalizedMethod) at System.Net.Http.HttpConnection.SendAsync(HttpRequestMessage request, Boolean async, CancellationToken cancellationToken) at System.Net.Http.HttpConnection.SendAsync(HttpRequestMessage request, Boolean async, CancellationToken cancellationToken) at System.Net.Http.HttpConnectionPool.SendWithVersionDetectionAndRetryAsync(HttpRequestMessage request, Boolean async, Boolean doRequestAuth, CancellationToken cancellationToken) at System.Net.Http.RedirectHandler.SendAsync(HttpRequestMessage request, Boolean async, CancellationToken cancellationToken) at Microsoft.Extensions.Http.Logging.LoggingHttpMessageHandler.

## 錯誤原因
HttpClient在一般情況下都是Thread-safe，但是如果有透過DefaultRequestHeaders處理header，則不是Thread-safe

## 解法
既然我又要設定Token，又要多執行續，只有一個辦法，一直new一個新的HttpClient

那麼，就有兩個方案，一個就是傳統的new HttpClient()
我選擇另一個方案，因為我有用DI注入，所以在註冊服務的時候，將原本的

 services.AddHttpClient("");

改為

 services.AddHttpClient("", (service, client) =>
 {
     client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", service.GetRequiredService<IOptions<AppConfig>>().Value.ApiToken);
 })